Remove skip_missing_interpreters

This prevents a job reporting 'success' when the appropriate python
interpreter cannot be found, when actually it didn't run at all.
Also change the default envlist to use generic 'py3' instead of a
specific version which might not be present.

Also change zuul config so the python-cinderclient-functional-py36
job runs on centos-8-stream nodes, where py36 should be available.
And change bindep.txt to specify the correct package name for
centos-8.

Jeremy Stanley has given a more thorough explanation of why this
is a good change:
http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014810.html

This isn't a theoretical issue.  If you look at recent
python-cinderclient-functional-py36 job results (for example,
[0]), you'll see that Zuul reported 'success', but on a closer look,
you'll see that no tests were run.

[0] https://zuul.opendev.org/t/openstack/build/1bfc80638086405f8b29905cdd6f71be/log/job-output.txt#25470

Change-Id: I2e2aa24e1592b66b287c84eda97b5079c40a36ec

.zuul.yaml

@@ -15,6 +15,8 @@
 - job:
     name: python-cinderclient-functional-py36
     parent: python-cinderclient-functional-base
+    # need to specify a platform that has python 3.6 available
+    nodeset: devstack-single-node-centos-8-stream
     vars:
       python_version: 3.6
       tox_envlist: functional-py36

bindep.txt

@@ -7,7 +7,7 @@ libffi-devel [platform:rpm]
 libssl-dev [platform:ubuntu-xenial]
 locales [platform:debian]
 python-dev [platform:dpkg]
-python-devel [platform:rpm]
+python-devel [platform:rpm !platform:centos-8]
 python3-all-dev [platform:ubuntu !platform:ubuntu-precise]
 python3-dev [platform:dpkg]
-python3-devel [platform:fedora]
+python3-devel [platform:rpm]

tox.ini

@@ -1,9 +1,8 @@
 [tox]
 distribute = False
-envlist = py36,py38,pep8
+envlist = py3,pep8
 minversion = 3.18.0
 skipsdist = True
-skip_missing_interpreters = true
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file
 ignore_basepython_conflict=true